Sub-aquatic contaminated sediments often represent a harmful and long term source of pollutants to the environment. A variety of approaches, such as dredging, have been used for the treatment of contaminated sediments, but they are expensive and can have limited value. Due to the increased volume of contaminated sediment cleanup projects both in the U.S. and abroad, sediment capping has become an option. In many areas the removal of material from a water body is not cost effective. In-situ capping of contaminated sediment is an efficient alternative that can have an immediate beneficial impact on the environment, as the contaminated sediment is isolated from aquatic organisms. Furthermore, capping contaminated sediments generally creates an anaerobic environment which permits natural degradation processes, which provide an opportunity for destruction and detoxification of harmful contaminants. Sediment capping has been used to contain harmful contaminants, including pesticides, metals, volatile organic compounds (VOCs), semi-volatile organic compounds (SVOCs), and polynuclear aromatic hydrocarbons (PAHs).
The capping of contaminated sediments is designed to prevent the upward migration of residual contaminants and/or to provide a clean subsurface bed of sediment that can be colonized by uncontaminated organisms. Capping alone could be used as a strategy to eliminate the need for dredging or could be used in conjunction with dredging to cover dredged locations with a clean layer of material where target clean-up goals cannot be achieved.
Previous methods of capping contaminated sediments have often involved mechanical equipment using buckets or direct slurry discharge into a water body. The mechanical bucket method often requires dumping large volumes of capping material into the water using a variety of buckets, including a clamshell bucket or dragline bucket. After releasing a bucket load it falls through a water column often as a distinct mass, which usually comes to rest on top of the contaminated material. This method has had some success in deep water producing caps with designed thickness over 12″. The water depth allows the capping material to disperse somewhat reducing velocity and concentration as it travels downward through the water. The thick cap design then accommodates the placement inaccuracies inherent in mechanical bucket placement.
The mechanical bucket method poses problems for relatively shallow water depth capping. When the mechanical bucket method is used to install thin layer caps (3″ to 12″), especially in shallow water (less than 10′), the results are often problematic. The capping material travels a relatively short distance through the water, thus causing its weight and velocity to displace the soft contaminated sediments. Displacement of the contaminated sediment is adverse to the purpose and goals of sediment capping. Furthermore, bucket placement of capping material leaves uneven mounds, which must then be raked in order to produce the proper thickness. This raking action often disturbs the underlying sediments, thereby causing sediment mixing and re-suspending of both the capping material and the contaminated sediments. The raking step can result in low production rates and capping material waste, and therefore higher production costs. In addition, bucket placement requires deep vessel draft requirements and cannot be employed in relatively shallow operations.
An alternative known capping method involves the open water slurry discharge method. Due to the large volume of water needed to transport the sand or gravel material this method also tends to displace the soft underlying material needing to be capped. Another problem with this method is that it requires sand or gravel slurry to be directly placed in water which raises turbidity levels. It would be advantageous for a sediment capping process to provide delivery of granular material from shallow draft vessels at relatively high rates of production with minimal disturbance of the sub-aquatic sediment.